Cam faces 24a and 24b are continuously formed in a circle on upper and lower faces which are perpendicular to an axial direction of a rotatable cam 24 which constitutes a rotational drive mechanism 21. Elastic members 23b are formed integrally with a holder member 23 for rotatably supporting the rotatable cam 24 to extend in the axial direction, and the first fixed cam and second fixed cam 23c and 23d having a small number of cam faces at butts and tips of the elastic members are arranged to face each other via the upper and lower cam faces 24a and 24b of the rotatable cam 24. The rotatable cam 24 is retreated and moved forward in the axial direction by writing pressure applied to a writing lead to be rotationally driven in one direction and rotational motion of the rotatable cam 24 is transmitted to the writing lead.